The role played by top-down knowledge can be seen in the “plumbing” of perception: in the case of vision, there are more downward fibers from the cortex to the brain’s relay stations than there are bottom-up from the eyes. So when we look at, say, a face, there is more data traveling downward from the knowledge areas of our brains than traveling upward from our eyes. Perception is what happens when the two interact.
